Method of full-color optical coherence tomography

ABSTRACT

Two-dimensional and three-dimensional optical coherence tomography is obtained by differential imaging of full-frame interference images using a white light source. Full-color tomographic imaging is also possible by processing the three-color channels of the interference images. A technique is described to obtain two-dimensional OCT images with full natural color representation. In a particular embodiment, the interference image is acquired using a color camera and the three-color channels are processed separately, recomposing the final image. In an additional embodiment, the interference images are acquired using separate red, blue and green light sources and the three color channels are combined to recompose the final image.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior application Ser. No.11/063,499 filed on Feb. 22, 2005, now U.S. Pat. No. 7,095,503 B2, thecontents of which are herein incorporated by reference, whichapplication claims the benefit of priority to provisional ApplicationNo. 60/546,408 filed Feb. 20, 2004, entitled “Method and Apparatus forFull-Color Two-Dimensional Optical Coherence Tomography,” and toprovisional Application No. 60/635,162 filed Dec. 10, 2004 entitled“Full-Color Three-Dimensional Microscopy by Wide-Field Optical CoherenceTomography.”

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under Grant No. DBI0243237 awarded by the National Science Foundation. The Government hascertain rights in the invention.

BACKGROUND OF INVENTION

In the field of biomedical imaging, various methods of optical imagingof biomedical tissues known in the art offer significant advantages overother biomedical imaging approaches. These advantages includenon-ionizing radiation, wide range of resolution, numerous and effectivecontrast mechanisms, and relatively compact and inexpensiveinstrumentation. Optical imaging can be applied to a wide range ofbiological systems, from cells and subcellular organelles, in vivo andex vivo tissues, to organs and whole body of a subject, covering thelength scales of nanometers and micrometers to millimeters andcentimeters. The contrast mechanisms include absorption, transmission,polarization, fluorescence, phase interference, nonlinear andmultiphoton processes, as well as time behavior of these processes.Another very important dimension in optical imaging is the effect ofthese processes as functions of the wavelength of the light—i.e. thespectroscopy of the images. Through spectral imaging, one can monitorfor detailed biochemical and biomedical parameters of the specimen.

Optical microscopy and its applications in biomedical imaging have beenexperiencing a remarkable growth over past few decades, thanks to suchtechnological developments as lasers and digital acquisition andprocessing of images. One of the main thrust areas of the development ofmodern microscopy is three-dimensional microscopy, where one acquires athree-dimensional image with every image plane sharply in focus. This isin contrast to conventional microscopy where the image of the in-focusplane is superposed with a blurred image of out-of-focus planes.

In confocal scanning microscopy, CSM, the out-of-focus signal isspatially filtered out by confocal aperturing of the object illuminationand the detector points. The three-dimensional image is constructed bypixel-by-pixel mechanical scanning of the entire object volume, whichplaces a fundamental limit on the image acquisition speed.

Another more recent development in 3D microscopy is optical coherencetomography (OCT), where the axial resolution of a few μm is provided byinterferometric measurement of the time-of-flight of short-coherencelight. In a typical arrangement, a Michelson-type interferometer isilluminated by femtosecond laser or superluminescent LED, and thereference arm is dithered to generate a heterodyne signal in theinterference with the back-scattered light from the sample point. Thetwo- or three-dimensional image is constructed from the mechanicalscanning over the sample area or volume, as in confocal scanningmicroscopy. In order to maintain the high speed of the z-scan, aconsiderably large depth of field, approximately a mm, is needed, whichcompromises the lateral resolution to a few μm. It is known in the artto sue a confocal adaptation to improve the resolution. The heterodynedetection allows for very high sensitivity and unique capabilities suchas Doppler velocity detection of blood flow. OCT-based imaging systemsare being developed for diverse areas of medical imaging includingretinal structures, endoscopy of gastrointestinal tract andcatheter-based intravascular imaging. As a coherent imaging technique,the OCT is capable of penetrating a larger distance into highlyscattering media such as biological tissues and ceramics.

Scanning microscopies, including the confocal microscopy and opticalcoherence tomography, have a number of distinct advantages such asrelaxed requirements on the imaging optics and high sensitivity and highresolution. On the other hand, the mechanical scanning is a majorlimiting factor in the image acquisition speed. Parallel acquisition oftwo-dimensional images while maintaining the optical sectioningcharacteristics of CSM or OCT would have obvious advantages. In CSM,such 2D imaging is approximated with a large number of well-spacedapertures, such as rotating Nipkow disk or multi-aperture scanning usingan electro-optic spatial light modulator, but the light efficiency orimage contrast tends to be low. Wide field optical sectioning is alsoachieved by structured light microscopy where a moving grating patternilluminates the object and processing of several images extracts thein-focus sectioned image.

With OCT, it is known that full-field interferometric images can beacquired using broadband light sources, and the regions of the imagethat do contain interferometric information can be extracted by digitalprocessing of the CCD images, thus generating optical section imagesvariously known as wide-field, full-field, or two-dimensional OCT. The3D image is constructed by mechanical scanning of the axial directiononly. In a simple 2DOCT system, the light source illuminates the entirearea of object to be imaged and its interference with the reference beamis imaged using a CCD array. For example, the reference optical pathlength is modulated by half wavelength, and pair of images is taken witha □ phase shift between them. The difference image then highlights theareas of interference within an axial depth equal to the coherencelength, while the rest of the image area is significantly attenuated. Aquasi-lock-in image acquisition is known in the art demonstratingsynchronous illumination, instead of synchronous detection, due to thelimited frame rate of the CCD camera, 30 Hz. High-frequency (50 kHz)true lock-in image acquisition has also been demonstrated in the artusing custom made smart array detector, although with a limited numberof pixels (58×58).

As such, the prior art demonstrates the potential for high-speedhigh-resolution 3D microscopy with very respectable sensitivity ordynamic range, at least ˜80 db, and promises to have significant impacton OCT applications where acquisition speed is critical, as in real-timein-vivo ophthalmic and endoscopic imaging.

Unlike conventional microscopy, most of the current developments of 3Dmicroscopy usually discard the natural color information of the object,whereas in some of the critical application areas of OCT such asophthalmic and dermatological imaging, the color and texture ofrelatively thin top layers of the tissues can provide vital informationin a format that is familiar to medical specialists in these areas.

Accordingly, what is needed in the art is a full-color 3D microscopicimaging system and method utilizing wide-field optical coherencetomography resulting in a 3D image with full natural colorrepresentation.

However, in view of the prior art considered as a whole at the time thepresent invention was made, it was not obvious to those of ordinaryskill in the pertinent art how the identified need could be fulfilled.

SUMMARY OF INVENTION

In accordance with the present invention a novel method of opticaltomography for surface and sub-surface imaging of biomedical tissues isprovided. The method is based on the principle of wide field opticalcoherence tomography (OCT) and is capable of providing full-colorthree-dimensional views of a tissue structure with high-resolution andhigh frame rate.

In a particular embodiment, the three-dimensional profile and structureof microscopic objects are obtained by processing CCD images of anobject in an interferometric apparatus.

In an additional embodiment, a novel method of full colortwo-dimensional optical coherence tomography is provided for surface andsub-surface imaging of biomedical tissues. The method as described isbased on the principle of wide field optical coherence tomography and iscapable of providing full-color two-dimensional and three-dimensionalimages of a tissue structure with high-resolution and high frame rate.

In an exemplary embodiment, contour or tomographic images are obtainedwith a phase locked interferometric imaging system using broadband lightsources and tri-color cameras. The interferometric images are analyzedin the three-color channels and recombined to generate 3D microscopicimages of tissue structures with full natural color representation. Incontrast to most existing biomedical optical tomographic methods, theproposed technique allows monitoring of tissue structures close to itsnatural color, and can provide critical information on the physiologicaland pathological states of tissues, such as in the retinal tissuesaffected by glaucoma.

In another exemplary embodiment in accordance with the presentinvention, full-color 3D microscopic imaging by WFOCT (wide-fieldoptical coherence tomography) is provided using three color LED's. TheWFOCT imaging is performed on an object three times using red, green,and blue LED's, and the resulting three RGB channels of the 3D imagesare combined to generate a final 3D image with full natural colorrepresentation.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent application file contains at least one drawing executed incolor. Copies of the patent or patent application publication with colordrawings will be provided by the Office upon request and payment of thenecessary fee. For a fuller understanding of the invention, referenceshould be made to the following detailed description, taken inconnection with the accompanying drawings, in which:

FIG. 1 is an exemplary for color wide-field optical coherence tomographyin accordance with the present invention;

FIG. 2 is an illustration of phase-shift interference imaging in whichfour quadrature phase interferograms and the extracted interferencesimages are shown, as well as cross-sectional profiles of theinterferograms;

FIG. 3( a) is an illustration of the spectra of red, green and blue LEDsand FIG. 3( b) is an illustration of the interference profiles of theLEDs vs. the axial distance z;

FIG. 4 is an illustration of the phase-shift interference imaging of acoin surface in accordance with the present invention in which, a) is adirect image of the object; b) is an image of the object with thereference wave; c) is an illustration of the contour image extracted bythe phase-shift interference; d) is a flat view of the accumulatedcontour images and where the image volume=12 mm×9 mm×405 μm;voxels=640×480×82; voxel volume=19 μm×19 μm×5 μm;

FIG. 5 is an illustration of the color WFOCT of a painted coin surfacein accordance with the present invention;

FIG. 6 is an illustration of the color WFOCT movies of a painted coinsurface in which a) (1.17 MB) xy-section images; b) (0.26 MB) xz-sectionimages; c) (0.90 MB) 3D perspective views and where the image volume=7.2mm×7.2 mm×335 μm; voxels=480×480×67; voxel volume=15 μm×15 μm×5 μm;

FIG. 7 is an illustration of a monochrome WFOCT movies of a bee in whicha) (0.73 MB) xy-section images; b) (0.26 MB) xz-section images; c) (0.64MB) 3D perspective views; d) direct image of the specimen and where theimage volume=6.0 mm×7.8 mm×980 μm; voxels=400×520×99; voxel volume=15μm×15 μm×10 μm;

FIG. 8 is an illustration of color WFOCT movies of an insect wing inwhich a) (0.81 MB) xy-section images; b) (0.38 MB) xz-section images; c)(0.78 MB) 3D perspective views; d) direct image of the specimen andwhere the image volume=7.2 mm×9.7 mm×810 μm; voxels=480×640×82; voxelvolume=15 μm×15 μm×10 m;

FIG. 9 is an illustration of a color WFOCT of a piece of apple skin inaccordance with the present invention;

FIG. 10 is an illustration of a color WFOCT movies of apple skin inwhich a) (0.27 MB) xy-section images; b) (0.10 MB) xz-section images; c)(0.90 MB) 3D perspective views and where the image volume=4.7 mm×4.7mm×170 μm; voxels=313×313×34; voxel volume=15 μm×15 μm×5 μm;

FIG. 11 is an illustration of a color WFOCT movies of a leaf in which a)(0.48 MB) xy-section images; b) (0.15 MB) xz-section images; c) (0.92MB) 3D perspective views and where the image volume=6.3 mm×6.3 mm×145μm; voxels=420×420×30; voxel volume=15 μm×15 μm×5 μm; and

FIG. 12 is an illustrative example of images obtained in accordance withthe system and method of the present invention, the images include colorWFOCT movies of a leaf, wherein a) (0.065 MB) xy-section images; b)(0.22 MB) xz-section images; c) (0.93 MB) 3D perspective views andwherein image volume=7.2 mm×7.2 mm×190 cm; voxels=480×640×39; voxelvolume=15 μm×15 μm×5 μm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings, which form a parthereof, and within which are shown by way of illustration specificembodiments by which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the invention.

The principle of color 3D microscopy by wide-field optical coherencetomography (WFOCT) is described referring to the diagram of theapparatus illustrated in FIG. 1. A high-brightness LED (˜30 lumens)illuminates the Michelson interferometer through a collimating lens L1,a polarizer P, and the broadband polarizing beam splitter BS. Thequarterwave plates Q1 and Q2 in the object or reference arms change theorthogonal polarization states, so that all of the reflections from theobject or the reference mirror are steered toward the monochrome CCDcamera through the imaging lens L2. The analyzer A combines the twobeams into a common polarization to affect the interference betweenthem. The combination of the polarizer and the analyzer allowscontinuous adjustment of the relative intensity and the interferencecontrast between the object and reference beams. The computer acquiresthe CCD image using an image acquisition board and a function generatoris used to strobe the LED and to dither the reference mirror mounted ona piezo-transducer. The object is mounted on a three-axis micrometertranslation stage to bring the appropriate object plane into focus.

The reference mirror is positioned at an equal distance with respect tothe object focal plane within the coherence length of the LED, and theobject contour is imaged by phase-shift interferometry. The lightintensity distribution I(x,y) on the CCD array is written as:I=I _(O)(x,y)+I _(B)(x,y)+I _(R)(x,y)+2√{square root over (I_(O)(x,y)I_(R)(x,y))}{square root over (I_(O)(x,y)I _(R)(x,y))}cos[φ_(i)+φ(x,y)]  (1)where I_(O)(x, y) is the portion of the object reflection that iscoherent with respect to the reference I_(R)(x,y) and I_(B)(x,y) is theincoherent background. φ(x,y) is the relative phase profile between theobject and the reference beams, whereas φ_(i) is the phase shiftintroduced by the piezo-driven reference. The reference mirror isdithered by a sawtooth wave applied to the reference piezo mount over afew micron amplitude. Four image frames are acquired at quarterwavelength intervals corresponding to reference phases of φ_(i)=0, λ/2,λ, and 3λ/2. These interference images are expressed as:I _(O) =I _(O) +I _(B) +I _(R)+2√{square root over (I_(O) I _(R))}cos φI _(π/2) =I _(O) +I _(B) +I _(R)−2√{square root over (I_(O) I _(R))}sinφI _(π) =I _(O) +I _(B) +I _(R)−2√{square root over (I_(O) I _(R))}cos φI _(3π2) =I _(O) +I _(B) +I _(R)+2√{square root over (I_(O) I _(R))}sinφ  (2)

The amplitude of the coherent object image is then given by

$\begin{matrix}{I_{0} = \frac{\left( {I_{0} - I_{\pi}} \right)^{2} + \left( {I_{\pi/2} - I_{3{\pi/2}}} \right)^{2}}{16I_{R}}} & (3)\end{matrix}$and the phase map

$\begin{matrix}{\varphi = {\tan^{- 1}\frac{I_{3{\pi/2}} - I_{\pi/2}}{I_{0} - I_{\pi}}}} & (4)\end{matrix}$

In FIG. 2, a plane mirror is used as the object, tilted by a small anglewith respect to the optical axis. The four interference images show thatthe fringes are shifted by quarter periods. The interference profilesare also plotted in FIG. 2. The bottom image and curve show the envelopeof the interference profile as obtained by Eq. (3). The coherence lengthof the light source can be determined by comparing the width of theinterference profile with the fringe periods. They are found to beδ_(R)=5.8 μm, δ_(G)=4.8 μm, and δ_(B)=4.0 μm for the three red, green,and blue LED's, respectively. The normalized spectra and theinterference profiles of the LED's are shown in FIG. 3. The centerwavelengths of the LED's are λR=638.4 nm, λ_(G)=528.5 nm, andλ_(B)=457.4 nm, while the spectral widths are Δ_(R)=22.9 nm, Δ_(G)=33.2nm, and Δ_(B)=28.6 nm. The measured coherence lengths are consistentwith the expected values of (2 ln 2/π)λ²/Δ within a factor of 1.3.

FIG. 4 illustrates the WFOCT by phase-shifting interferometry using acoin surface as the test object. FIG. 4 a) is the direct image of theobject in the absence of the reference wave, while FIG. 4 b) is when thereference wave is present. Although the two images are mostlyindistinguishable, one can observe fluctuating speckles in portions ofthe object when the camera image is viewed in real time while thereference mirror is dithered. The coherent portion of the image isextracted as described above to obtain a contour of the object at aheight that corresponds to the reference mirror position, as shown inFIG. 4 c). The contour imaging is repeated a number of times (˜40) andaveraged to improve the signal-to-noise ratio. The three dimensionalimaging is completed by stepping the object z-position over a desiredrange to obtain a stack of the contour images. The three-dimensionalimage can then be presented in a number of different ways. In FIG. 4 d)all the contour images are added together resulting in a ‘flat view’,where all the image planes are in focus and additionally the overallhaze in FIG. 4 a) or b) due to stray reflections from various opticalsurfaces has been removed.

In an exemplary embodiment, for generation of color images in accordancewith the present invention, the WFOCT procedure is repeated three timesby using red, green, and blue LED's. Each of these generates athree-dimensional image of the object under the respective colorillumination. The three 3D images are then combined as RGB channels togenerate the final 3D image with full natural color representation ofthe volume.

FIG. 5 and FIG. 6 illustrate experimental results in accordance with anembodiment of the present invention in which a colored coin is used asthe test object. The backside of a dime is painted with red, green, andblue ink and the background is painted white. The first three columns ofFIG. 5 represent separate results obtained using the three differentLED's. In each column, the top square is a 7.2 mm×335 μm xz-section viewof the 3D image, the middle square is a 7.2 mm×7.2 mm xy-section or acontour image, while the bottom square is the flat view for the colorchannel. The last column is the RGB composite images. In FIG. 6, the 3Dimage is presented in a few different modes. FIG. 6 a) is a series ofxy-sections and FIG. 6 b) is a series of xz-sections. In FIG. 6 c) thereconstructed 3D image is viewed from varying perspective angles. Noticethe strong color-independent (white) reflection at the air-paintinterface and strong colored reflection from underneath. Penetrationdepth in some areas is at least about 100 μm. In the x-z RGB image, topright corner of FIG. 5, the thin and abrupt layer of red probablyaccounts for thin layer of paint on the metal surface, whereas there isa pool of white paint in the valley next to it. Also note that the RGBflat image has very respectable color and image quality, compared to thedirect photographic image in the lower right corner of FIG. 5.

Two examples of biological imaging by monochrome WFOCT are shown withreference to FIG. 7 and FIG. 8. FIG. 7 is an image of the body of a beeand FIG. 8 is an image of the wing of an unidentified insect. The XY-and XZ-section movies start with the flat view followed by a series ofcross-sectional tomographic views in the respective directions. Theimage volumes are several millimeters in the lateral direction and lessthan a millimeter in the longitudinal direction. The relatively opaquebody of the bee is seen to have a penetration depth of about 100 μm orso, whereas it is a few hundred microns for the more transparent wing.The typical signal to noise ratio of these images has been measured tobe between approximately 50 and 60 db.

Examples of color 3D microscopy using the three-channel WFOCT inaccordance with the present invention are illustrated with reference toFIG. 9. The images shown in FIG. 9 are arranged in the same manner asthose identified in FIG. 5, with the XZ- and XY-sectional views and theflat view for each of the three color channels and the RGB compositecolor images, for a 4.7 mm×4.7 mm×170 μm volume of a piece of appleskin. Here the penetration depth is at least 150 μm, and it displaysdetails of the top surface and the skin tissue. Because of the largedifference in the longitudinal and lateral scales, the minute surfacetexture is seen greatly exaggerated in the flat views. The crosssectional views clearly show the outer skin layer and a portion of theinternal tissue layers. One can also notice the correspondence of theinternal structure of a blemish area in the x-z sections and flat viewimages. The extra picture in the lower right corner is a directphotograph of the approximate area of the imaging experiment. Althoughthe color variation of the apple skin is rather subtle and continuouscompared to the colored coin experiment, the RGB flat view reproducesthe color variation at least approximately. In this data set the coloredimages are not as one would expect from common macroscopic views of anapple skin. However, under a microscope, the coloring of apple skinappears not as a solid layer of red surface, but the red pigments areembedded as numerous particles in otherwise colorless matrix. Still oneobserves some variation of color that can be attributed to the actualcolor variation of the surface. FIG. 10 illustrates the movies for XY-and XZ-sections and the 3D images of the data set.

In FIGS. 11 and 12, the color WFOCT in accordance with the presentinvention is used to generate color 3D images of plant leaves. In FIG.11, one clearly sees the pattern of pores on the surface as well as aminor stem. A careful examination of the interior of the leaf also showsa fibrous structure running the length of the leaf. In FIG. 12, a pieceof scotch tape was present over a part of the leaf in the imaged area.The 3D imaging system clearly imaged through this slightly diffusinglayer. As illustrated by these exemplary embodiment and experimentalresults, the overall quality of the images acquired using the method andapparatus in accordance with the present invention are quite convincingin terms of the resolution, signal level, and the overall capability togenerate 3D images with close to natural color information.

The present invention provides a system and method to generatethree-dimensional microscopic images with natural color representation.The present invention provides the ability to generate images ofbiological specimens with about 10 μm axial resolution, about 100-200 μmpenetration depth, and 50-60 db dynamic range. These images aregenerated with close to natural color representation.

Optical section (or tomographic) imaging and imaging through turbidmedia are very active areas of current research. The method of fullcolor WFOCT microscopy in accordance with the present invention can beused for the generation of optical section images in a wide range offields of study including biology, medicine, geology, material sciences,and microelectronics. Optical sectioning or tomographic capabilitiesallow generation of images with the entire 3D volume of the objectsharply in focus. The color imaging techniques provided by the method ofthe present invention provide additional critical information in many ofthese application areas. Some of the foreseeable applications inbiomedicine will be in the imaging of the eyes, the optical biopsy ofexternal tissues, and epithelial layers of gastrointestinal andpulmonary tissues.

It will be seen that the advantages set forth above, and those madeapparent from the foregoing description, are efficiently attained andsince certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatters contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween. Now that theinvention has been described,

1. A method of generating an object image by optical coherence tomography with natural color representation, the method comprising the steps of: performing optical coherence tomography imaging on the object illuminated with a red LED resulting in a first image; performing optical coherence tomography imaging on the object illuminated with a blue LED resulting in a second image; performing optical coherence tomography imaging on the object illuminated with a green LED resulting in a third image; combining the first, second and third images to generate a fourth image having full natural color representation; wherein the step of performing optical coherence tomography imaging on the object illuminated with a red LED further comprises the steps of: illuminating the object with a reference wave; performing phase-shift interference imaging resulting in four image frames acquired at quarter wavelength intervals of the reference wave; extracting the first image from the resulting four image frames; and wherein the object of interest is biological tissue. 